The present invention relates to barrier membranes and, more particularly, to barrier membranes which, under certain embodiments, serve to selectively control the diffusion of gases through the membrane. Additionally, under certain embodiments, the membrane not only selectively controls the diffusion of gases through the membrane, but also allows for the controlled diffusion of gases normally contained in the atmosphere.
For a further understanding of the scope of the present invention, reference can be made to U.S. patent application Ser. No. 08/299,287, entitled xe2x80x9cCushioning Device With Improved Flexible Barrier Membranexe2x80x9d which was filed on Aug. 31, 1994 and allowed; U.S. patent application Ser. No. 08/299,286 entitled xe2x80x9cLaminated Resilient Flexible Barrier Membranesxe2x80x9d which was filed on Aug. 31, 1994 and abandoned; and U.S. patent application Ser. No. 08/475,276, entitled xe2x80x9cBarrier Membranes Including A Barrier Layer Employing Polyester Polyolsxe2x80x9d which is commonly owned and was filed on Jun. 1, 1995 and abandoned; each of the aforementioned patent applications being expressly incorporated herein by reference.
Barrier membranes useful for containing fluids, including liquids and/or gases, in a controlled manner have been employed for years in a wide variety of products ranging from bladders useful in inflatable objects, such as vehicle tires and sporting goods for example; to accumulators used on heavy machinery; to cushioning devices useful in footwear. Regardless of the intended use, barrier membranes must generally be flexible, resistant to environmental degradation and exhibit excellent gas transmission controls. Often, however, materials which exhibit acceptable flexibility characteristics tend to have an unacceptably low level of resistance to gas permeation. In contrast, materials which exhibit an acceptable level of resistance to gas permeation tend to have an unacceptably low level of flexibility.
In an attempt to address the concerns of both flexibility and imperviousness to gases, U.S. Pat. No. 5,036,110 which issued Jun. 30, 1991, to Moreaux describes resilient membranes for fitting hydropneumatic accumulators. According to Moreaux ""110, the membrane disclosed consists of a film formed from a graft polymer which is the reaction product of an aromatic thermoplastic polyurethane with a copolymer of ethylene and vinyl alcohol (EVOH), with this film being sandwiched between layers of thermoplastic polyurethane to form a laminate. While Moreaux ""110 attempts to address the concerns in the art relating to flexibility and imperviousness to gases, a perceived drawback of Moreaux is that the film described is not processable utilizing conventional sheet extrusion techniques. Thus, the present invention is directed to barrier membranes which are flexible, have good resistance to gas transmission, and under certain embodiments are processable into laminates utilizing conventional sheet extrusion techniques which are highly resistant to delamination.
While it should be understood by those skilled in the art upon review of the following specification and claims that the barrier membranes of the present invention have a broad range of applications, including but not limited to bladders for inflatable objects such as footballs, basketballs, soccer balls and inner tubes; films for food packaging; as well as the production of fuel lines and fuel storage tanks to name a few, still other highly desirable applications include their use in forming accumulators which are operable under high pressure environments.
For convenience, but without limitation, the barrier membranes of the present invention will hereinafter generally be described in terms of either accumulators or in terms of still another highly desirable application, namely for cushioning devices used in footwear. In order to fully discuss the applicability of the barrier membranes in terms of cushioning devices for footwear, a description of footwear in general is believed to be necessary.
Footwear, or more precisely, shoes generally include two major categories of components namely, a shoe upper and the sole. The general purpose of the shoe upper is to snugly and comfortably enclose the foot. Ideally, the shoe upper should be made from an attractive, highly durable, yet comfortable material or combination of materials. The sole, which also can be made from one or more durable materials, is particularly designed to provide traction, protect the wearer""s feet and body during use which is consistent with the design of the shoe. The considerable forces generated during athletic activities require that the sole of an athletic shoe provide enhanced protection and shock absorption for the feet, ankles and legs of the wearer. For example, impacts which occur during running activities can generate forces of up to 2-3 times the body weight of an individual while certain other activities such as, for example, playing basketball have been known to generate forces of up to approximately 6-10 times an individual""s body weight. Accordingly, many shoes and, more particularly, many athletic shoes are now provided with some type of resilient, shock-absorbent material or shock-absorbent components to cushion the user during strenuous athletic activity. Such resilient, shock-absorbent materials or components have now commonly come to be referred to in the shoe manufacturing industry as the midsole.
It has therefore been a focus of the industry to seek midsole designs which achieve an effective impact response in which both adequate shock absorption and resiliency are appropriately taken into account. Such resilient, shock-absorbent materials or components could also be applied to the insole portion of the shoe, which is generally defined as the portion of the shoe upper directly underlining the plantar surface of the foot.
A particular focus in the shoe manufacturing industry has been to seek midsole or insert structure designs which are adapted to contain fluids, in either the liquid or gaseous state, or both. Examples of gas-filled structures which are utilized within the soles of shoes are shown in U.S. Pat. No. 1,900,867 entitled xe2x80x9cCushion for Footwearxe2x80x9d which issued Oct. 13, 1908, to Miller; U.S. Pat. No. 1,069,001 entitled xe2x80x9cCushioned Sole and Heel for Shoesxe2x80x9d which issued Jul. 29, 1913, to Guy; U.S. Pat. No. 1,304,915 entitled xe2x80x9cPneumatic Insolexe2x80x9d which issued May 27, 1919, to Spinney; U.S. Pat. No. 1,514,468 entitled xe2x80x9cArch Cushionxe2x80x9d which issued Nov. 4, 1924, to Schopf; U.S. Pat. No. 2,080,469 entitled xe2x80x9cPneumatic Foot Supportxe2x80x9d which issued May 18, 1937, to Gilbert; U.S. Pat. No. 2,645,865 entitled xe2x80x9cCushioning Insole for Shoesxe2x80x9d which issued Jul. 21, 1953, to Towne; U.S. Pat. No. 2,677,906 entitled xe2x80x9cCushioned Inner Sole for Shoes and Method of Making the Samexe2x80x9d which issued May 11, 1954, to Reed; U.S. Pat. No. 4,183,156 entitled xe2x80x9cInsole Construction for Articles of Footwearxe2x80x9d which issued Jan. 15, 1980, to Rudy; U.S. Pat. No. 4,219,945 entitled xe2x80x9cFootwearxe2x80x9d which issued Sep. 2, 1980, also to Rudy; U.S. Pat. No. 4,722,131 entitled xe2x80x9cAir Cushion Shoe Solexe2x80x9d which issued Feb. 2, 1988, to Huang; and U.S. Pat. No. 4,864,738 entitled xe2x80x9cSole Construction for Footwearxe2x80x9d which issued Sep. 12, 1989, to Horovitz. As will be recognized by those skilled in the art, such gas filled structures often referred to in the shoe manufacturing industry as xe2x80x9cbladdersxe2x80x9d typically fall into two broad categories, namely (1) xe2x80x9cpermanentlyxe2x80x9d inflated systems such as those disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 and (2) pump and valve adjustable systems as exemplified by U.S. Pat. No. 4,722,131. By way of further example, athletic shoes of the type disclosed in U.S. Pat. No. 4,182,156 which include xe2x80x9cpermanentlyxe2x80x9d inflated bladders have been successfully sold under the trade mark xe2x80x9cAir Solexe2x80x9d and other trademarks by Nike, Inc. of Beaverton, Oreg. To date, millions of pairs of athletic shoes of this type have been sold in the United States and throughout the world.
The permanently inflated bladders are typically constructed under methods using a flexible thermoplastic material which is inflated with a large molecule, low solubility coefficient gas otherwise referred to in the industry as a xe2x80x9csuper gas,xe2x80x9d such as SF6. By way of example, U.S. Pat. No. 4,340,626 entitled xe2x80x9cDiffusion Pumping Apparatus Self-Inflating Devicexe2x80x9d which issued Jul. 20, 1982, to Rudy, which is expressly incorporated herein by reference, discloses a pair of elastomeric, selectively permeable sheets of film which are formed into a bladder and thereafter inflated with a gas or mixture of gases to a prescribed pressure which preferably is above atmospheric pressure. The gas or gases utilized ideally have a relatively low diffusion rate through the selectively permeable bladder to the exterior environment while gases such as nitrogen, oxygen and argon which are contained in the atmosphere and have a relatively high diffusion rate are able to penetrate the bladder. This produces an increase in the total pressure within the bladder, by the addition of the partial pressures of the nitrogen, oxygen and argon from the atmosphere to the partial pressures of the gas or gases contained initially injected into the bladder upon inflation. This concept of a relative one-way addition of gases to enhance the total pressure of the bladder is now known as xe2x80x9cdiffusion pumping.xe2x80x9d
Under the diffusion pumping system and depending upon the bladder material used and the choice of gas or gases contained therein, there is a period of time involved before a steady state of internal pressure is achieved. For example, oxygen tends to diffuse into the bladder rather quickly with the effect being an increase in the internal pressure of approximately 2.5 psi. In contrast, over the course of a number of weeks nitrogen gas will gradually diffuse into the bladder resulting in an increase of pressure to approximately 12.0 psi. The gradual increase in bladder pressure typically causes an increase in tension in the bladder skin, resulting in a volume increase due to stretching. This effect is commonly referred to in the industry as xe2x80x9ctensile relaxationxe2x80x9d or xe2x80x9ccreep.xe2x80x9d Thus, it is of significant importance which materials are chosen for the bladder and the choice of the captive gas mixture utilized to initially inflate the bladder to achieve a bladder which is essentially permanently inflated at a desired internal pressure and which maintains a desired internal pressure over an extended period of time.
With regard to the systems utilized within the shoe manufacturing industry prior to and shortly after the introduction of the Air Sole(trademark) athletic shoes, many of the midsole bladders consisted of a single layer gas barrier type films made from polyvinylidene chloride based materials such as Saran(copyright) (which is a registered trademark of the Dow Chemical Co.) and which by their nature are rigid plastics, having relatively poor flex fatigue, heat sealability and elasticity. Still further, bladder films made under techniques such as laminations and coatings which involve one or more barrier materials in combination with a flexible bladder material (such as various thermoplastics) can potentially present a wide variety of problems to solve. Such difficulties with composite constructions include layer separation, peeling, gas diffusion or capillary action at weld interfaces, low elongation which leads to wrinkling of the inflated product, cloudy appearing finished bladders, reduced puncture resistance and tear strength, resistance to formation via blow-molding and/or heat-sealing and R-F welding, high cost processing, and difficulty with foam encapsulation and adhesive bonding, among others.
Yet another issue with previously known bladders is the use of tie-layers or adhesives in preparing laminates. The use of such tie layers or adhesives generally prevent regrinding and recycling of any waste materials created during product formation back into an usable product, and thus, also contribute to high cost of manufacturing and relative waste. These and other short comings of the prior art are described in more extensive detail in U.S. Pat. Nos. 4,340,626; 4,936,029 and 5,042,176, all of which are hereby expressly incorporated by reference.
With the extensive commercial success of the products such as the Air Sole(trademark) shoes, consumers have been able to enjoy a product with a long service life, superior shock absorbency and resiliency, reasonable cost, and inflation stability, without having to resort to pumps and valves. Thus, in light of the significant commercial acceptance and success that has been achieved through the use of long life inflated gas filled bladders, it is highly desirable to develop advancements relating to such products. The goal then is to provide flexible, xe2x80x9cpermanentlyxe2x80x9d inflated, gas-filled shoe cushioning components which meet, and hopefully exceed, performance achieved by such products as the Air Sole(trademark) athletic shoes offered by Nike, Inc.
One key area of potential advancement stems from a recognition that captive gases other than the large molecule, low solubility coefficient xe2x80x9csuper gasesxe2x80x9d as described in the ""156, ""945 and ""738 patents utilized can be replaced with less costly and possibly more environmentally friendly gases. For example, U.S. Pat. Nos. 4,936,029 and 5,042,176 specifically discuss the methods of producing a flexible bladder film that essentially maintains permanent inflation through the use of nitrogen as the captive gas. As further described in U.S. Pat. No. 4,906,502, also specifically incorporated herein by reference, many of the perceived problems discussed in the ""029 and ""176 patents are solved by the incorporation of mechanical barriers of crystalline material into the flexible film such as fabrics, filaments, scrims and meshes. Again, significant commercial success for footwear products using the technology described in ""502 patent under the trademark Tensile Airs(trademark) sold by Nike, Inc. has been achieved. The bladders utilized therein are typically comprised of a thermoplastic urethane laminated to a core fabric three-dimensional, double bar Raschel knit nylon fabric, having SF6 as the captive gas contained therein.
By way of example, an accepted method of measuring the relative permeance, permeability and diffusion of different film materials is set forth in the procedure designated as ASTM D-1434-82. According to ASTM D-1434-82, permeance, permeability and diffusion are measured by the following formulas:       Permeance    ⁢          xe2x80x83                          (                  quantity          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          gas                )                              (          area          )                xc3x97                  (          time          )                xc3x97                  (                      press            .                          xe2x80x83                        ⁢            diff            .                    )                      =                  Permeance                              (            GTR            )                    /                      (                          press              .                              xe2x80x83                            ⁢              diff                        )                              =                        cc          .                                      (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                    ⁢                      (            Pa            )                                    Permeability    ⁢          xe2x80x83                                    (                      quantity            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            gas                    )                xc3x97                  (                      film            ⁢                          xe2x80x83                        ⁢            thick                    )                                      (          area          )                xc3x97                  (          time          )                xc3x97                  (                      press            .                          xe2x80x83                        ⁢            diff            .                    )                      =                  Permeability                              (            GTR            )                    xc3x97                                    (                              film                ⁢                                  xe2x80x83                                ⁢                thick                            )                        /                          (                              press                .                                  xe2x80x83                                ⁢                diff                            )                                          =                                    (            cc            )                    ⁢                      (            mil            )                                                (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                    ⁢                      (            Pa            )                                    Diffusion    ⁢          xe2x80x83                          (                  quantity          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          gas                )                              (          area          )                xc3x97                  (          time          )                      =                            Gas          ⁢                      xe2x80x83                    ⁢          Transmission          ⁢                      xe2x80x83                    ⁢          Rate                          (          GTR          )                    =              cc                              (                          sq              .                              xe2x80x83                            ⁢              m                        )                    ⁢                      (                          24              ⁢                              xe2x80x83                            ⁢              hr                        )                              
By utilizing the above listed formulas, the gas transmission rate in combination with a constant pressure differential and the film""s thickness, can be utilized to define the movement of gas under specific conditions. In this regard, the preferred gas transmission rate (GTR) for a bladder in an athletic shoe component which seeks to meet the rigorous demands of fatigue resistance imposed by heavy and repeated impacts has a gas transmission rate (GTR) value of approximately 10.0 or lower and, even more preferably, a (GTR) value of 2.0 or lower, for bladders having an average thickness of approximately 20 mils.
In addition to the aforementioned, the ""029 and ""176 patents also discuss problems encountered with previous attempts to use co-laminated combinations of plastic material which operate as barriers to oxygen. In this regard, the principal concern was the lack of fatigue resistance of the barrier layer. As described in the ""176 patent, a satisfactory co-lamination of polyvinylidene chloride (such as Saran(copyright)) and a urethane elastomer would require an intermediate bonding agent. Under such a construction, relatively complicated and expensive processing controls such as strict time-temperature relationships and the use of heated platens and pressures, coupled with a cold press to freeze the materials together under pressure would be required. Additionally, using adhesive tie layers or incorporating crystalline components into the flexible film at high enough levels to accomplish a gas transmission rate of 10.0 or less, reduces the flexibility of the film.
Cushioning devices which specifically eliminate adhesive tie layers have been known to separate or de-laminate especially along seams and edges. Thus, it has been a relatively recent focus of the industry to develop cushioning devices which reduce or eliminate the occurrence of delamination, ideally without the use of a xe2x80x9ctie layer.xe2x80x9d In this regard, the cushioning devices disclosed in co-pending U.S. application Ser. Nos. 08/299,286 and 08/299,287 eliminate adhesives tie layers by providing membranes including a first layer of thermoplastic urethane and a second layer including a copolymer of ethylene and vinyl alcohol wherein hydrogen bonding occurs over a segment of the membranes between the first and second layers. While the cushioning devices disclosed in U.S. patent application Ser. No. 08/299,287 and the laminated flexible barrier membranes of U.S. patent application Ser. No. 08/299,286 are believed to offer a significant improvement in the art, still further improvements are offered according to the teachings of the present invention.
It is therefore, a principal object of the present invention to provide barrier membranes which offer enhanced flexibility, durability and resistance to the undesired transmission of fluids therethrough.
It is another object of the present invention to provide barrier membranes which can essentially be permanently inflated with nitrogen or another environmentally desirable gas or combination of gases wherein the barrier membrane provides for a gas transmission rate value of 10.0 or less, based on a 20 mils average thickness.
It is still another object of the present invention to provide barrier membranes and, particularly those employed as cushioning devices with improved clarity and consistency.
It is yet another object of the present invention to provide barrier membranes which can be formed into laminated objects such as cushioning devices or accumulators which resist delamination and do not require a tie layer between the barrier layer and the flexible layers.
It is yet another object of the present invention to provide barrier layers which are reprocessable.
It is a further object of the present invention to provide barrier membranes which are formable utilizing the various techniques including, but not limited to, blow-molding, tubing, sheet extrusion, vacuum-forming, heat-sealing and RF welding.
Still another object of the present invention is to provide barrier membranes which prevent gas from escaping along interfaces between the layers in laminated embodiments and particularly along seems via capillary action.
It is yet another object of the present invention to provide a barrier membrane which allows for normal footwear processing such as encapsulation within a formable material.
While the aforementioned objects provide guidance as to possible applications for the barrier membranes of the present invention, it should be recognized by those skilled in the art that the recited objects are not intended to be exhaustive or limiting.
To achieve the foregoing objects, the present invention provides barrier membranes which have (1) a desirable level of flexibility (or rigidity); (2) a desirable level of resistance to degradation caused by moisture; (3) an acceptable level of imperviousness to fluids which can be in the form of gases, liquids or both depending mainly on the intended use of the product; and (4) are highly resistant to delamination when employed in a multi-layer structure. Regardless of the barrier membrane embodiment, each barrier membrane in accordance with the teachings of the present invention includes a barrier layer comprised at least in part of a blend of at least one aliphatic thermoplastic urethane and at least one copolymer of ethylene and vinyl alcohol.
The aliphatic thermoplastic urethanes employed, if not commercially available, are generally formed as the reaction product of (a) at least one polyester and/or polyether diol; (b) at least one difunctional extender, and (c) at least one aliphatic isocyanate and/or diisocyanate, as will be described in greater detail below. Additionally, it may be desirable to utilize a catalyst to activate the reaction and one or more processing aids.
The term xe2x80x9cthermoplasticxe2x80x9d as used herein is intended to mean that the material is capable of being softened by heating and hardened by cooling through a characteristic temperature range, and as such in the softened state can be shaped into various articles under various techniques.
The term xe2x80x9cpolyester diolxe2x80x9d as used herein is intended to preferably mean polymeric polyester diols having a molecular weight (determined by the ASTM D-4274 method) falling in the range of about 300 to about 4,000; more preferably from about 400 to about 2,000; and still more preferably between about 500 to about 1,500.
The term xe2x80x9cpolyether diolxe2x80x9d as used herein is intended to preferably mean polymeric polyether diols having a molecular weight (determined by ASTM D-4274 method) falling in the range of about 300 to about 6,500; more preferably from about 400 to about 4,500; and still more preferably between about 500 to about 3,000.
The term xe2x80x9cdifunctional extenderxe2x80x9d is used preferably in the commonly accepted sense to one skilled in the art and includes glycols, diamines, amino alcohols and the like having a molecular weight generally falling in the range of from about 60 to about 300.
The terms xe2x80x9caliphatic isocyanatexe2x80x9d and xe2x80x9caliphatic diisocyanatexe2x80x9d as used herein are intended preferably to mean linear aliphatic, cycloaliphatic and hindered aromatic isocyanates and diisocyanates where the isocyanate xe2x80x94Nxe2x80x94 is separated from the benzenoid ring proper by at least one carbon, and hence, acts to produce light stable or xe2x80x9caliphaticxe2x80x9d (poly)urethanes.
Ideally, the flexible barrier materials utilized in accordance with the teachings of the present invention should be capable of containing a captive gas for a relatively long period of time. In a highly preferred embodiment, for example, the barrier membrane should not lose more than about 20% of the initial inflated gas pressure over a period of two years. In other words, products inflated initially to a steady state pressure of between 20.0 to 22.0 psi should retain pressure in the range of about 16.0 to 18.0 psi.
Additionally, the barrier materials utilized should be flexible, relatively soft and compliant and should be highly resistant to fatigue and be capable of being welded to to form effective seals typically achieved by RF welding or heat sealing. The barrier material should also have the ability to withstand high cycle loads without failure, especially when the barrier material utilized has a thickness of between about 5 mils to about 50 mils. Another important characteristic of the barrier membrane is that they should be processable into various shapes by techniques used in high volume production. Among these techniques known in the art are extrusion, blow molding, injection molding, vacuum molding, rotary molding, transfer molding and pressure forming. The barrier membranes of the present invention should be preferably formable by extrusion techniques, such as tubing or sheet extrusion, including extrusion blow molding particularly at sufficiently high temperatures to attain the desired xe2x80x9cadhesivexe2x80x9d or xe2x80x9cchemicalxe2x80x9d bonding as will be described in greater detail below. These aforementioned processes should give rise to products whose cross-sectional dimensions can be varied.
As alluded to above, a significant feature of the barrier membranes of the present invention is the ability under embodiments formed into products intended to be inflated (such as cushioning devices for footwear) to control diffusion of mobile gases through the membrane and to retain the captive gases contained therein. By the present invention, not only are super gases usable as captive gases, but nitrogen gas may also be used as a captive gas due to the performance of the barrier. The practical effect of providing a barrier membrane for which nitrogen gas is a captive gas is significant in terms of protection of the earth""s ozone and global warming.
Under the present invention, if the barrier membrane is formed into a product such as a cushioning device, the membrane may be initially inflated with nitrogen gas or a mixture of nitrogen gas and one or more super gases or with air. If filled with nitrogen or a mixture of nitrogen and one or more super gases, an increment of pressure increase results from the relatively rapid diffusion of oxygen gas into the membrane, since the captive gas is essentially retained within the membrane. This effectively amounts to an increase in pressure of not greater than about 2.5 psi over the initial inflation pressure and results in a relatively modest volume growth of the membrane of between 1 to 5%, depending on the initial pressure. However, if air is used as the inflatant gas, oxygen tends to diffuse out of the membrane while the nitrogen is retained as the captive gas. In this instance, the diffusion of oxygen out of the membrane and the retention of the captive gas results in an incremental decrease of the steady state pressure over the initial inflation pressure.
A further feature of the present invention is the enhanced bonding which occurs between contiguous layers, thus, eliminating the need for adhesive tie layers. This is generally accomplished by laminating the first and second layers together using conventional techniques and thus, the laminated barrier membranes of the present invention are characterized in that significant hydrogen bonding occurs between a first layer formed from a blend of at least one aliphatic thermoplastic urethane and a copolymer of ethylene and vinyl alcohol, and a second layer of thermoplastic urethane. In addition to the occurrence of hydrogen bonding, it is theorized that there will also generally be a certain amount of covalent bonding between the first and second layers, especially when lesser amounts of the copolymer of ethylene and vinyl alcohol is used in the first layer and the thermoplastic urethanes of both the first and second layers have similar functionalities.
This invention has many other advantages which will be more apparent from consideration of the various forms and embodiments of the present invention. Again, while the embodiments shown in the accompanying drawings which form a part of the present specification are illustrative of embodiments employing the barrier membranes of the present invention, it should be clear that the barrier membranes have extensive application possibilities. Various exemplary embodiments will now be described in greater detail for the purpose of illustrating the general principles of the invention, without considering the following detailed description in the limiting sense.